Control systems are often employed in association with conveyor systems for moving objects along guided tracks, including modular conveyor sections or “sticks”. Conveyor systems for moving objects between stations in a manufacturing environment or for accumulating and distributing products in a warehouse operation are well known in the art. Such conveyor systems provide upwardly exposed conveying surfaces, such as rollers, positioned between guiding side rails. The rollers can be powered by controllable motors to move objects placed on top of the rollers along a track defined by the rails.
Assembly of conveyor systems can be facilitated by employment of “conveyor sticks” which may include one or more short sections of rollers and guide rails, which are connected together to form a final conveyor system. The conveying surface of each conveyor stick may be broken up into one or more zones, respective zones associated with a sensor for detecting the presence of an object on the conveyor at the zone. A control circuit communicates with the zones and associated sensors via a number of cables to control the zones, in order to accomplish a number of standardized tasks. Such conveyor systems may be adapted to perform one or more tasks or operations. One such task is that of “accumulation” in which a control circuit for a given zone operates its rollers when the sensor, in an adjacent “upstream” zone, indicates an object is at that zone and the sensor of an adjacent “downstream” zone indicates that no object is in that downstream zone. This logic causes the conveyor zones to move objects along to fill adjoining zones with objects. Generally, each upstream control circuit operates its rollers to move its objects downstream one zone. In order to perform these tasks, the control circuit for a particular conveyor stick may communicate in a limited fashion with the control circuits (or at least the sensors) of an associated, adjacent upstream and downstream conveyor stick. This may be accomplished via cabling between control cards or sensors of the conveyor sticks, typically within one of the side rails.
Several problems currently exist with conventional distributed zone control systems, however. One such problem relates to transmission line issues (e.g., reflections, noise) as a plurality of control stations can be concatenated for larger conveyor lines. Other problems relate to cable and associated installation expenses when adding additional stations to an existing line or in the initial design and installation of the conveyor line itself. This can be caused by the amount of different types of sensors, actuators and controllers that have to be interconnected to form a cohesive system. Still yet another problem involves speed and smoothness during conveyor operations. Due to communications limitations between zones, conveyor speed generally must be limited to avoid causing instabilities in the overall conveyor and associated control process.
Employing a centralized controller over all the zones can alleviate some of the control and stability issues described above. Industrial controllers are special purpose computers utilized for controlling industrial processes, manufacturing equipment, and other factory automation, such as conveyor systems. In accordance with a control program, the industrial controller measures one or more process variable or inputs reflecting the status of a controlled conveyor system, and changes outputs effecting control of the conveyor system. The inputs and outputs may be binary, (e.g., on or off), as well as analog inputs and outputs assuming a continuous range of values. The control program may be executed in a series of execution cycles with batch processing capabilities.
Measured inputs received from a conveyor system and the outputs transmitted to the conveyor system generally pass through one or more input/output (I/O) modules. These I/O modules serve as an electrical interface between the controller and the conveyor system, and may be located proximate or remote from the controller. The inputs and outputs may be recorded in an I/O table in processor memory. Input values may be asynchronously read from the controlled conveyor system by one or more input modules and output values are written directly to the I/O table by the processor for subsequent communication to the conveyor system by specialized communications circuitry. An output module may interface directly with a conveyor system, by providing an output from an I/O table to an actuator such as a motor, valve, solenoid, and the like.
Various control modules of the industrial controller may be spatially distributed along a common communication link in several racks. Certain I/O modules may thus be located in close proximity to a portion of the control equipment, and away from the remainder of the controller. Data is communicated with these remote modules over a common communication link, or network, wherein modules on the network communicate via a standard communications protocol. Although centralized industrial controllers can be effective in controlling a conveyor line, these type solutions can add significant expense to a conveyor system. These expenses include the controller such as a Programmable Logic Controller (PLC), associated racks, I/O modules, communications modules, program software development, and extensive cabling to facilitate centralized control of a distributed conveyor system.